Knowledge of the junction temperature of an LED can be useful to control the output of the LED. Typically, the peak wavelength and perceived colour of an LED depends on the operating temperature. Furthermore the luminous intensity of an LED can vary with temperature.
Although it is possible to measure the temperature of the LED, the temperature in the critical region—that is to say, the junction, which is typically a pn junction for a conventional LED—is generally not directly accessible. In the case that the LED is mounted on a heatsink, a temperature sensor may typically be mounted on the heatsink, and the temperature of the LED estimated from the measured heatsink temperature, by generating a thermal model of the arrangement, in order to estimate a temperature off-set between the measured heatsink temperature and the LED junction.
Recently, techniques have been developed by the applicant to directly estimate the junction temperature of an LED, based on its electrical characteristics. Such techniques do not require use of a separate temperature sensor, and may be referred to a “sensor-less sensing”. In particular it has been recognised that LED current-voltage characteristic (“IV curve”) of an LED has a well-defined relationship with temperature. Even a single measurement of the voltage across an LED at a specified operating current may thus be used to estimate the LED junction temperature, according to the well-known diode equation:
  I  =            I      0        ·                  (                              e                                          q                ·                                  (                                      V                    -                                          I                      ·                                              R                        S                                                                              )                                            nkt                                -          1                )            .      
The applicant has refined these techniques to provide an estimation of the junction temperature from the voltage across the LED at a high current, and the voltage across the LED at a relatively lower current. Typically, the high current may be of the order of 10 mA to 10 A, and the relatively lower current may be of the order of 100 μA or less.
In particular, in applications where it is desirable to measure the temperature of a plurality of LEDs, the processing power required to apply such techniques might be considerable and not readily available. Such applications include use cases where an array of LEDs is required with matched operating wavelengths, or applications in which differently coloured LEDs (for instance, red, green and blue—RGB, or red, green blue and white—RGBW) are combined to provide a specific luminosity and perceived colour.